Inkjet printers are well known in the art. A typical inkjet printhead comprises a silicon substrate, structures built on the substrate, and connections to the substrate to provide an array of drop generators. Such a printhead typically uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). Each drop generator comprises a precisely formed orifice or nozzle attached to the substrate that incorporates an ink ejection chamber that receives liquid ink from an ink reservoir. Each chamber is located opposite a nozzle so ink can collect between it and the nozzle. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to ink ejectors (typically, resistor elements) on the substrate. When electric printing pulses activate the ink ejectors (i.e., heat a resistor element), a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged drop generators form a dot matrix pattern. Properly sequencing the operation of each drop generator causes characters or images to be printed upon paper as the printhead moves past the paper.
Given current manufacturing technology, drop generators provided on silicon-based printheads of the type described above are currently capable of producing only a single drop weight. That is, the characteristics of each drop generator (i.e., chamber volume, orifice layer thickness, etc.) on a printhead are essentially identical. As a result, the drop weight or drop volume provided by each drop generator will be substantially identical. Furthermore, different types of inks have different dot gains, i.e., the amount of coverage provided for an equivalent ink drop volume. For example, different ink colors typically required different drop weights to produce equivalent amounts of coverage. In particular, black inks generally require a drop weight about twice that of color inks in that they have much different spreading characteristics to achieve substantially equal pixel coverage.
Given these circumstances, it is not currently possible to combine black and color architectures on a single printhead. One possible solution to this problem would be to provide equivalent drop generator structures for black and color inks and activate the black drop generators more frequently (i.e., twice as often) than the color drop generators, thereby providing similar coverage. However, this approach would effectively halve the expected reliability of black drop generators relative to color. Thus, the current approach to combine color and black inks in a single printer is to provide multiple print cartridges; one for each color (typically: cyan, yellow and magenta) and one for black. While this approach works, a cost and competitive advantage would be provided if black and color architectures could be combined into a single printhead and, therefore, a single cartridge.